As people age, their eyes are less able to accommodate, or bend the natural lens, to focus on objects that are relatively near to the observer. This condition is known as presbyopia. Contact lenses can be worn to address presbyopia. In one type of such lenses, distance and near vision regions are concentrically arranged around the geometric center of the lens. Light passing through the optical zone of the lens is concentrated and focused at more than one point in the eye.
In another type of lens, a segmented lens, near and distance vision regions are not concentric about the geometric center of the lens. The wearer of the segmented lenses is able to access the near vision region of the lens because the lens is constructed to allow it to translate, or move vertically relative to the pupil of the wearer's eye. The lens moves vertically when the person wearing the lens shifts their gaze downwardly to read. This upwardly positions the near vision portion in the center of the wearer's gaze. Substantially all of the light passing through the optical zone can be focused at a single point in the eye based on gaze.
One type of translating lens has a truncated shape. That is, unlike most lenses that are continuously circular or oval, the lower portion of the truncated contact lens is flattened by cutting off or shortening that part of the lens. This results in a substantially flat, thick edge at the bottom of the lens. Exemplary descriptions of such lenses include U.S. Pat. No. 7,543,935; U.S. Pat. No. 7,430,930; U.S. Pat. No. 7,052,132; U.S. Pat. No. 4,549,794 incorporated herein by reference. Unfortunately, a relatively flat edge on contact lenses such as these tends to reduce comfort. It is desirable to have a translating contact lens with improved comfort.
Another type of translating lens has an outer shape which is continuously circular or oval, but contains a substantially thickened portion peripheral to the central optical zone. This thickened portion is intended to contact the lower lid and translate with blink. Exemplary references to such a lens are described in U.S. Pat. No. 7,040,757 and US 20100171924, incorporated herein by reference. In these examples, the thickness in the peripheral portions of the lens outside the optical zone is substantially uniform for meridians parallel to the vertical meridian of the lens, and a lens according to this invention exhibits mirror symmetry with respect to a plane cutting through the vertical meridian.
U.S. Pat. No. 7,216,978 shows that the upper and lower eyelids do not move strictly in a vertical, with an up and down stroke during blink. The upper lid moves substantially vertical, with a small nasal component during blink, and the lower lid moves substantially horizontal, moving nasalward during blink. Additionally, the upper and lower eyelids are not symmetrical with respect to a plane cutting through the vertical meridian.
Lens surfaces can be generated using different functions. For example, U.S. Pat. No. 3,187,338, and U.S. Pat. No. 5,975,694 describe a sine function; U.S. Pat. No. 6,843,563 uses a third order polynomial function, and U.S. Pat. No. 5,650,838 uses a tangent function; in U.S. Pat. No. 6,540,353 a lens surface is generated using a rapid power change over a small distance in the optical zone and in U.S. Pat. No. 5,608,471, a rapid transition on the lens surface is made by a straight linear function.
In U.S. Pat. No. 7,004,585, the distance and near centers of a translating lens both lie on the vertical bisector of the optical zone.
It would be advantageous to have a contact lens with a feature that fully engage the lower eyelid of the wearer to facilitate lens translation and which provides improved wearing comfort.